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On the afternoon of February 24, 2012, at 5:15 p.m. EST local time, a United Launch Alliance Atlas V rocket lifted off from the pad at Cape Canaveral Air Force Base carrying in its payload the US Navy’s next-generation narrowband communications satellite MUOS-1. After two scrubbed launches the previous week due to weather, the third time was definitely a charm for ULA, and the launch went nominally (that’s science talk for “awesome”.)
But what made that day, that time the right time to launch? Do they just like ending a work week with a rocket launch? (Not that I could blame them!) And what about the weather… why go through the trouble to prepare for a launch at all if the weather doesn’t look promising? Where’s the logic in that?
As it turns out, when it comes to launches, it really is rocket science.
There are a lot of factors involved with launches. Obviously all the incredible engineering it takes to even plan and build a launch vehicle, and of course its payload — whatever it happens to be launching in the first place. But it sure doesn’t end there.
Launch managers need to take into consideration the needs of the mission, where the payload has to ultimately end up in orbit… or possibly even beyond. Timing is critical when you’re aiming at moving targets — in this case the targets being specific points in space (literally.) Then there’s the type of rocket being used, and where it is launching from. Only then can weather come into the equation, and usually only at the last minute to determine if the countdown will proceed before the launch window closes.
How big that launch window may be — from a few hours to a few minutes — depends on many things.
Kennedy Space Center’s Anna Helney recently assembled an article “Aiming for an Open Window” that explains how this process works:
The most significant deciding factors in when to launch are where the spacecraft is headed, and what its solar needs are. Earth-observing spacecraft, for example, may be sent into low-Earth orbit. Some payloads must arrive at a specific point at a precise time, perhaps to rendezvous with another object or join a constellation of satellites already in place. Missions to the moon or a planet involve aiming for a moving object a long distance away.
For example, NASA’s Mars Science Laboratory spacecraft began its eight-month journey to the Red Planet on Nov. 26, 2011 with a launch aboard a United Launch Alliance (ULA) Atlas V rocket from Cape Canaveral Air Force Station in Florida. After the initial push from the powerful Atlas V booster, the Centaur upper stage then sent the spacecraft away from Earth on a specific track to place the laboratory, with its car-sized Curiosity rover, inside Mars’ Gale Crater on Aug. 6, 2012. Due to the location of Mars relative to Earth, the prime planetary launch opportunity for the Red Planet occurs only once every 26 months.
Additionally, spacecraft often have solar requirements: they may need sunlight to perform the science necessary to meet the mission’s objectives, or they may need to avoid the sun’s light in order to look deeper into the dark, distant reaches of space.
Such precision was needed for NASA’s Suomi National Polar-orbiting Partnership (NPP) spacecraft, which launched Oct. 28, 2011 aboard a ULA Delta II rocket from Vandenberg Air Force Base in California. The Earth-observing satellite circles at an altitude of 512 miles, sweeping from pole to pole 14 times each day as the planet turns on its axis. A very limited launch window was required so that the spacecraft would cross the ascending node at exactly 1:30 p.m. local time and scan Earth’s surface twice each day, always at the same local time.
All of these variables influence a flight’s trajectory and launch time. A low-Earth mission with specific timing needs must lift off at the right time to slip into the same orbit as its target; a planetary mission typically has to launch when the trajectory will take it away from Earth and out on the correct course.
According to [Eric Haddox, the lead flight design engineer in NASA's Launch Services Program], aiming for a specific target — another planet, a rendezvous point, or even a specific location in Earth orbit where the solar conditions will be just right — is a bit like skeet shooting.
“You’ve got this object that’s going to go flying out into the air and you’ve got to shoot it,” said Haddox. “You have to be able to judge how far away your target is and how fast it’s moving, and make sure you reach the same point at the same time.”
But Haddox also emphasized that Earth is rotating on its axis while it orbits the sun, making the launch pad a moving platform. With so many moving players, launch windows and trajectories must be carefully choreographed.
It’s a fascinating and complex set of issues that mission managers need to get just right in order to ensure the success of a launch — and thus the success of a mission, whether it be putting a communication satellite into orbit or a rover onto Mars… or somewhere much, much farther than that.